The production of polyolefins in fluidized beds requires that the heat of reaction be removed in order to maintain appropriate temperatures for the desired reaction rate. In addition, the temperature of the vessel cannot be permitted to increase to the point where the product particles become sticky and adhere to each other. The heat of reaction is commonly removed by circulating the gas from the fluidized bed to a heat exchanger outside the reactor and passing the cooled gas back to the reactor.
For example, fluidized bed processes for producing polymer resins are practiced by passing a gaseous stream containing one or more monomers continuously through a fluidized bed reactor under reactive conditions in the presence of a polymerization catalyst. Product is withdrawn from the reactor. A gaseous stream of unreacted monomer is withdrawn from the reactor continuously and recycled into the reactor along with make-up monomer added to the recycle stream.
In condensing mode processes, the recycle stream, or a portion thereof, is cooled to a temperature below the dew point, resulting in condensing all or a portion of the recycle stream. The recycle stream is returned to the reactor. The dew point of the recycle stream can be increased by increasing the operating pressure of the reaction/recycle system and/or increasing the percentage of condensable fluids and decreasing the percentage of non-condensable gases in the recycle stream. Increasing the percentage of condensable fluids can be achieved by introducing an induced cooling agent to the reactor. Greater quantities of heat energy are removed in less time which increases the production capacity of the typical exothermic fluidized bed reactor. Also known in the art is the so-called “induced condensed mode” and “liquid monomer” methods.
Liquid monomer polymerization mode may also be employed, as disclosed in U.S. Pat. No. 5,453,471 which is hereby incorporated by reference. When operating in the liquid monomer mode, liquid can be present throughout the entire polymer bed provided that the liquid monomer present in the bed is adsorbed on or absorbed in solid particulate matter present in the bed, such as polymer being produced or fluidization aids (e.g., carbon black) present in the bed. The liquid monomer process comprises introducing a stream of one or more monomers and optionally one or more inert gases or liquids into the polymerization zone; introducing a polymerization catalyst into the polymerization zone; withdrawing polymer product from the polymerization zone; withdrawing unreacted gases from the zone; and compressing and cooling the gases while maintaining the temperature within the zone below the dew point of at least one monomer present in the zone. If there is only one monomer present in the gas-liquid stream, preferably there also is present at least one inert gas.
The increase in production capacity should be balanced by the amount of induced cooling agent used. The induced cooling agent is a fluid which is inert to the catalyst, reactants and the products of the polymerization reaction. The induced cooling agent is sometimes called an induced condensing agent (ICA). The amount of induced cooling agent should be optimized to lower manufacturing costs, reduce the possibility of reactor fouling, and reduce the environmental impact. There is a need for providing a method for optimizing the amount of induced cooling agent used while maximizing the production capacity of the fluidized bed reactor.